Hello everyone,
I also want to share my experience with my MacMini 2018 with i7 and 512GB SSD. I bought this little fellow February this year, and from the beginning I was experiencing jumpy Magic Mouse behavior and random mouse and keyboard disconnects. As the time was running and I was adding more devices (MIDI stuff, cameras, USB hubs) to MacMini, the mouse became to move in completely unacceptable choppy stuttering way.
Unfortunately we become too much dependent on this device (and macOS) to return it to the store or wait for repair that might be not even possible in this case. I started looking for a solution, first blaming Intel built-in GPU as too weak drive my 4K screen, bad batteries and dirty battery contacts in Magic Mouse - but replacing batteries and cleaning contacts did not help. Almost bought new Magic Mouse, yeah almost paid Apple again!, but thank God I found this thread.
Instead I bought $20 Startech bluetooth dongle that has CSR chip supported out of the box by Mac. And now yes it works great! Mouse movement is finally butter smooth.
But this is modern Apple world... so things cannot be THAT easy or configurable. Plugging USB dongle in does not work - yes the device is recognized, but you need to use Bluetooth Explorer developer tool with its HCI host switch functionality to set the dongle as active Bluetooth host device. There is NO other way. And of course this selection is not preserved after the reboot. Because in Apple's wisdom, why anyone would want to use 3rd party bluetooth device instead of Apple flawlessMagicBluetooth(tm)!
At least
sudo nvram bluetoothHostControllerSwitchBehavior=always advertised on few sites as a way to make USB dongle preferred instead of built-in one did not work for me on Mojave 10.14.6. Simply after the reboot I had no connection with Apple's mouse & keyboard and I had to use extra USB keyboard/mouse to log in, just to see that built-in dongle was again active.
Obviously I filed a ticket to Apple, but I don't expect any solution as I did not get any for many many other ticket. No single word. What I expect is "closed as duplicate" as I got that for many other bugs I have reported, that were closed as duplicate, but linked bugs still remain unresolved.
Now going back to having Dongle active after the reboot, I had to do a little macOS programming exercise and additionally trace Bluetooth Explorer and its private API. All to create a little tool here:
https://github.com/nanoant/blueswitch
It ensures that your dongle is re-selected as active after the reboot. After wasting yet another 4 hours now I have functional MacMini mouse! Feel free to use it, share it, do whatever you want with these few lines of code and rattled off README. Just please don't ask me for details how to compile/use it as I am completely fed up with this subject